Positron Annihilation Spectroscopy: Polymer Blends and Miscibility

Broadly classified, there are three types of polymer blends, namely, (1) miscible polymer blends, (2) immiscible polymer blends, and (3) partially miscible polymer blends. There are many different experimental methods that can be used to investigate the miscibility of polymer blends, such as differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), dielectric measurement, cloud point measurement, microscopy, light scattering, small-angle X-ray scattering, small-angle neutron scattering, fluorescence technique, and nuclear magnetic resonance (NMR) spectroscopy. Each of these experimental methods can only probe the homogeneity (or heterogeneity) in a polymer blend at a certain scale range. Thus, the determination of the miscibility in polymer blends depends on the resolution limit of the experimental method(s) employed. For instance, DSC and DMTA have frequently been used to determine the miscibility in polymer blends by determining glass transition temperature Tg. When a single Tg value is observed in a polymer blend, the blend can be considered miscible. However, there is a general consensus among researchers that such an experimental criterion, while very useful, cannot guarantee that a polymer blend is miscible on a segmental level. Therefore, a serious question may be raised as to whether a polymer pair can be regarded as being miscible on the segmental level (say, less than approximately 5 nm). It has been reported that DMTA can resolve the size of domains (or separated phases) on the order of 5–10 nm (Molnar and Eisenberg 1992) and DSC is not as sensitive as DMTA for determining the Tg of a polymer blend (Stoelting et al. 1970). In the use of DSC to investigate the miscibility of polymer blends, one often encounters the situation where a very broad (say, 40–60 ◦C) single glass transition appears for certain blend compositions, such as polystyrene/poly(α-methyl styrene) (PS/PαMS) blends (Kim et al. 1998; Lin and Roe 1988; Saeki 1983) and polystyrene/poly(vinyl methyl ether) (PS/PVME) blends (Kim et al. 1998; Schneider and Wirbser 1990; Schneider et al. 1990). Under such circumstances, it is not clear how an unambiguous, single value of Tg can be read off from a DSC thermogram.

Dynamic coordination of miscible polymer blends towards highly designable shape memory effect

Soda lime silicate glass produced by the float process is among the materials with the largest use worldwide. The manufacturing process introduces an asymmetry in the glass sheets that causes well‐known modifications in terms of properties between the so‐called “air side” and “tin side”. Nevertheless, not much is known about the structural changes induced by the diffusing tin ions in the network. In the present work, we show that surprisingly, the short range of the tin side is akin to that of the bulk, while the network connectivity in the air side vicinity is enhanced with a larger concentration of bridging oxygens. This correlates with a partial depletion of modifiers from the air side as detected by secondary ion mass spectrometry. On the other hand, the incorporation of tin markedly changes the medium range. In fact, the tin side is characterized by: (i) longer correlation distances, (ii) a more homogeneous structure depicted by a more reduced splitting of the medium frequency Raman features into R and R c bands, (iii) a substantially lower amount of free volume, and (iv) a slight reduction in the average free volume size. Furthermore, the spontaneous hydration layer on the two surfaces is studied, and its effect on the free volume is discussed. These results provide new insight into the soda lime silicate glass structure and have various technological implications for processes like chemical tempering or phenomena like surface defects nucleation.

Dielectric study on the miscibility of binary polymer blends

Free volume study of 70 MeV carbon induced modification in polymers through positron annihilation

We tested forward recoil spectrometry (FRES) as a method to determine miscibility by measuring coexistence compositions in binary polymer blends. In this study, equilibrium phase compositions were determined for a compositionally symmetric poly(styrene-ran-methyl methacrylate) random copolymer (S0.49-r-MMA) and two homopolymers, deuterated polystyrene (dPS) and deuterated poly(methyl methacrylate) (dPMMA). Sample preparation, film dewetting, and beam damage were addressed, and the results for these polymer blends were in good agreement with those obtained through other experimental techniques. Deuteration had a strong effect on the miscibility of the dPS/S0.49-r-MMA and dPMMA/S0.49-r-MMA blends, to the extent that the asymmetric miscibility observed separately for the PS/S0.49-r-MMA and PMMA/S0.49-r-MMA blends was not found. Although this deuteration effect may limit the applicability of FRES for some polymer systems, the accuracy with which phase compositions can be determined with FRES makes it an attractive alternative to other less quantitative methods for investigating blend miscibility. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1547–1552, 2000

Polymer blends were prepared to obtain a new material with specific characteristics. To prepare a miscible polymer blend, it is necessary to evaluate the chemical structure and molecular dynamics of all polymers involved and to thereby establish a structure–property relationship for these materials. In this work, ternary blends for application in plates chromatography were formed with styrene/butadiene/acrylinotrile (ABS), poly(vinyl chloride) (PVC), and gypsum, and were studied by solid-state carbon-13 nuclear magnetic resonance (13C NMR). Magic-angle spinning (MAS), cross-polarization magic-angle spinning (CPMAS), variable contact time (VCT), and proton spin-lattice relation time in the rotating frame (Tρ) were the techniques used to evaluate the miscibility, homogeneity, and compatibility of polymer blends at the molecular level. From the values of Tρ, it was verified that ABS/PVC/gypsum formed good polymer blends with some specific physical interaction at the molecular level. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 293–296, 2003

The theoretical modeling of the relaxation behavior of polymers in the glass transition region, advocated by Moynihan and co-workers, has been used to analyze the heat flow and the relaxation of polymer systems during isothermal modulated DSC experiments in the glass transition region. An analytic solution for the frequency dependent fictive temperature is obtained, which takes a particularly simple form in the high-frequency region. The maximal phase lag of the fictive temperature Tf is βπ/2, where the exponent of the stretched-exponential characterizing the enthalpy relaxation, β, is on the order of 0.1−0.7. The corresponding maximal phase lag in the heat flow is much smaller, on the order of 2−5 deg. It is once more iterated that, as observed long ago by Birge and Nagel, the loss heat capacity corresponds to the entropy production due to a redistribution of energy over the heat baths. The possibility of using specific-heat spectroscopy as a tool to determine miscibility in polymer blends whose constitu...

Nanostructural aspects, free volume and phase constitution of rapidly solidified Nd–Fe–B

Spinodal nanostructures in polymer blends: On the validity of the Cahn-Hilliard length scale prediction

Free volume pockets inside a cell membrane play a prominent role in a variety of dynamic processes such as the permeability of small molecules across membranes and the diffusion of, e.g., lipids, drugs, and electron carriers in the plane of the membrane. Nonetheless, by now the chances for characterizing free volume voids in a nonperturbative manner through experiments have been very limited. Here we use lipid membranes as an example to show how positron annihilation spectroscopy (PALS) together with atomistic simulations can be employed to gauge changes in free volume pockets in biological macromolecular complexes. The measurements show that PALS is a viable technique to probe free volume in biomolecular systems. As examples, we consider the gel-to-fluid transition and the role of increasing cholesterol concentration in a lipid membrane. Further applications proposed in this work for PALS are likely to provide a great deal of insight into the understanding of the role of free volume in the dynamics of biomolecular complexes.

The free volume of metallic glasses has a significant effect on atomic relaxation processes, although a detailed understanding of the nature and distribution of free volume sites is currently lacking. Positron annihilation spectroscopy was employed to study free volume in a Zr–Ti–Ni–Cu–Be bulk metallic glass following plastic straining and cathodic charging with atomic hydrogen. Multiple techniques were used to show that strained samples had more open volume, while moderate hydrogen charging resulted in a free volume decrease. It was also shown that the free volume is associated with zirconium and titanium at the expense of nickel, copper, and beryllium. Plastic straining led to a slight chemical reordering.

It is well known that structural relaxation in a bulk metallic glass affects some properties such as viscosity, electrical conductivity, and ductility. The free volume in glassy alloy, which is open volume retained by rapidly solidification from liquid state, has a significant roll for those properties. In order to discuss the nature of free volume in a Zr50Cu40Al10 bulk metallic glass, positron lifetime measurements have been performed for this metallic glass before and after annealing, and all positron lifetime spectra have been decomposed into multi components as a size distribution function by use of CONTIN-PALS II program. The positron lifetime distribution for this metallic glass has a broad spectrum comparing to crystal metal including a single vacancy, and its average lifetime corresponds to the free volume size. This width of positron lifetime (free volume size) distribution decreases by the annealing at 673 K. This change of the positron lifetime distribution can be attributed to a free volume relaxation.

Modification of microstructure of the surface and the bulk in ion-irradiated membrane studied using positron annihilation spectroscopy

The topochemical synthesis of a miscible polymer blend is described. The azide- and alkyne-decorated tetrol 1 crystallizes as two different conformers. Both conformers exhibit self-sorted head-to-tail alignment with proximally placed reacting groups such that topochemical polymerization yields two types of polymer chains, each containing only one type of conformer. The orientation of complementary reactive groups in one of the head-to-tail-arranged conformers favors the formation of cis-triazole linkages, and the other favors the trans-triazole linkages. Crystals of 1 on heating gave a perfect polymer blend containing equal amounts of cis-triazole-linked and trans-triazole-linked polymers. As each conformer is H-bonded to four conformers of the other kind, the polymerization yields a perfect polymer blend wherein each polymer chain is surrounded by chains of the other type. Thus, the molecular ordering in the prepolymerized state in a crystal is utilized to create a polymer blend.